This application generally relates to a method and apparatus for the detection of a liquid under a surface. More particularly, a method of remotely detecting oil spilled under a layer of ice, snow, or water is disclosed herein.
This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Oil production or transfer in ice-prone marine or freshwater locations could result in a subsurface release—for example from a well blowout or leaking pipeline—that results in oil trapped beneath ice. Oil-spill countermeasures will require that this oil is accurately located and mapped. Currently, there is no effective technique to remotely sense the location and extent of a spill under ice. Characterizing ice (e.g. thickness, age, and extent) in these areas is also important for conducting hydrocarbon recovery and response operations, and determining potential impact loads on offshore structures.
The detection of oil under ice has been of concern since the exploration and production for hydrocarbon resources in the Arctic began in the early 1970's. There have been numerous attempts to detect oil under ice using acoustics, optical/UV excitation, and ground penetrating radar (for a review, see “Detection and Tracking of Oil under Ice”, D. F. Dickins, report submitted to the US Department of the Interior Minerals Management Service, Oct. 6, 2000). All of these techniques have shown the capability to detect oil under ice with some success; however, they have not been used in the field. The methods proposed to date have a limited range of applicability and are susceptible to false positive results. They also have only a limited ability to “see” or detect oil through a layer of ice and require contact with the ice surface.
Notably, all three methods require access and traverse across the ice surface, some require the removal of snow cover, and special care must be taken to ensure good ice contact with the sensor. The surface access limitation presents both logistic and safety concerns such as breakthrough, and limits the coverage to a small area per day.
Dickins, et al. (2006) successfully detected oil under ice using ground-penetrating radar using a skid-mounted unit pulled along the ice surface (see “2006 Experimental Spill to Study Spill Detection and Oil Behavior in Ice”, D. F. Dickins, P. J. Brandvik, L. G. Faksness, J. Bradfor, and L. Liberty; report submitted to the US Department of the Interior Minerals Management Service, Dec. 15, 2006, contract number 1435-0106-CT-3925). Tests with the system mounted in a helicopter were less conclusive although additional research is planned.
Up to this point, the only remote-detection technique to provide a direct signal from oil is the optical/UV detection method. However, this technique is limited to the detection of oil at the surface or only a few millimeters below the surface because the opaque nature and heterogeneity of both ice and snow scatter the luminescence to obscure the signal. Current techniques for detecting oil under ice are not effective or accurate enough to support a proper response in the event of an oil spill. In addition, these techniques are labor-intensive and dangerous due the surface contact requirement.
Ice characterization technology is similarly limited to ground penetrating radar (GPR), electromagnetic (EM), and satellite detection methods. GPR provides reliable thickness measurements for low salinity ice, but significant signal attenuation occurs for high-salinity first-year ice. EM methods measure ice plus snow thickness and require in-situ measurements. Satellite measurements in the infrared band only work for thin ice because surface temperature becomes less dependent on thickness for ice greater than 1.2 meters thick.
Nuclear magnetic resonance (“NMR”) is a tool used for the characterization of the molecular composition of liquids and solids. More particularly, in some applications NMR is used to distinguish between a solid (e.g. rock in the earth) and a liquid (e.g. ground water or oil). NMR molecular characterization works by placing a sample in a magnetic field to align the magnetic moments of the protons with the field. The proton magnetic moments are then perturbed using one or more radio-frequency (RF) excitation pulses. The energy released as these magnetic moments return to equilibrium is monitored by a receiver.
In the oil and gas industry, NMR is applied in reservoir characterization in the field for well logging measurements and in laboratory analysis of rock cores. The NMR logging tool technology is capable of directly detecting the signals from fluids in the rock pore space and differentiating between different types and phases of fluids. In well logging, a magnet and a radio-frequency generator/receiver is lowered into the bore hole. NMR well logging tools, such as those in commercial use by oilfield service companies such as Schlumberger, Halliburton and BakerHughes, detect fluids in the pore space over a volume on the order of several cubic decimeters (dm3). An example of such a tool is NUMAR™ by Schlumberger.
NMR has also been used to detect aquifers (e.g. an underground formation including ground water). Such instruments typically utilize the earth's magnetic field as the static magnetic field, detect a larger volume than the downhole devices (cubic meters (m3) rather than dm3), and are placed on the earth's surface for operation. Examples of such a system are NUMIS™ and NUMIS PLUS™ by Heritage Geophysics. These devices typically utilize a 100 meter diameter wire loop placed flat on the ground as the sending/receiving antenna. The large loop permits the sensing of aquifers over a larger volume than the downhole devices (m3 v. dm3) and greater depths (up to 150 m). A measurement time of about one hour per detection volume is typically required. Current NMR research in geophysical applications addresses difficulties that arise when attempting to identify liquids located in pores or at a surface between a liquid and a solid. See, e.g., PAPE, et al., Pore Geometry of Sandstone Derived from Pulsed Field Gradient NMR, J. of Applied Geophysics 58, pp. 232-252 (2006).
While NMR tools have been used for a variety of applications, the technology has not been used to detect oil spilled under a surface such as water, ice, or snow, nor has it been used to characterize ice.
Other useful information may be found in the following references: U.S. Pat. No. 3,019,383; U.S. Pat. No. 4,022,276; U.S. Pat. No. 4,769,602; U.S. Pat. No. 4,868,500; Gev, et al., Detection of the Water Level of Fractured Phreatic Aquifers Using Nuclear Magnetic Resonance (NMR) Geophysical Measurements, J. of Applied Geophysics 34, pp. 277-282 (1994); SLICHTER, CHARLES P., Principles of Magnetic Resonance, 2nd Edition Springer Series in Solid-State Sciences, (1996); LEGCHENKO, et al., Nuclear Magnetic Resonance as a Geophysical Tool for Hydrogeologists, J. of Applied Geophysics 50, pp. 21-46 (2002); WEICHMAN, et al., Study of Surface Nuclear Magnetic Resonance Inverse Problems, J. of Applied Geophysics 50, pp. MOHNKE, et al., Smooth and Block Inversion of Surface NMR Amplitudes and Decay Times Using Simulated Annealing, J. of Applied Geophysics 50, pp. 163-177 (2002); SHUSHAKOV, et al., Hydrocarbon Contamination of Aquifers by SNMR Detection, WM'04 Conference, Feb. 29-Mar. 4, 2004, Tucson, Ariz.